Why 'Hot Jupiter' Exoplanets Aren't Eaten by Their Stars

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Huge, scorching-hot alien planets are successful daredevils,
coming dangerously close to their parent stars but rarely being
consumed, a new study finds.

The inward migration of gas-giant exoplanets known
as "hot Jupiters" tends to halt before they spiral down to their
doom, researchers said.

"Eventually, all hot Jupiters get closer and closer to their
stars, but in this study we are showing that this process stops
before the stars get too close," lead author Peter Plavchan, of
NASA's Exoplanet Science Institute at Caltech in Pasadena, said
in a statement. "The planets mostly stabilize once their orbits
become circular, whipping around their stars every few days."
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Hot Jupiters are similar to planets such as Saturn and
Jupiter in mass and composition. These exotic worlds form
relatively far away from their stars, as our solar system's gas
giants did, but then move inward over time in a process that
remains poorly understood.

We're fortunate that Saturn and Jupiter didn't do this, for such
migrations can boot smaller, rocky planets out of the way,
sometimes ejecting them out of the solar system entirely,
scientists say.

What could halt inward migrations, allowing
hot Jupiters to survive in their close-in orbits, has also
been mysterious. Scientists have offered up three main theories,
with the first positing that the host star's magnetic field
serves as a barrier.

The second theory suggests that hot Jupiters stop migrating when
they reach a different roadblock — the end of the dusty part of
the newborn system's planet-forming disk.

"This theory basically said that the dust road a planet travels
on ends before the planet falls all the way into the star,"
co-author Chris Bilinski of the University of Arizona said in a
statement. "A gap forms between the star and the inner edge of
its dusty disk where the planets are thought to stop their
migration."

Plavchan and Bilinski put these theories to the test by studying
126 confirmed exoplanets, along with more than 2,300 candidate
worlds (most of them spotted by NASA's Kepler space telescope) of
many different sizes and orbital characteristics. Specifically,
they looked at how these planets' orbital distances varied
according to the masses of their host stars.

They found that hot Jupiters circling more massive stars orbited
relatively farther out, just as theory number three — the "orbit
circularization" idea — had predicted.

"When only a few hot Jupiters were known, several models could
explain the observations," Kepler scientist Jack Lissauer of
NASA's Ames Research Center in Moffet Field, Calif., who was not
affiliated with the study, said in a statement. "But finding
trends in populations of these planets shows that tides, in
combination with gravitational forces by often unseen planetary
and stellar companions, can bring these giant planets close to
their host stars."